Female fish with bigger brains choose better mates

It takes brains to choose a good partner. In one of the first experiments to look at the cognitive demands of choosing a mate, female guppies with big brains showed a preference for more colourful males, while those with smaller brains showed no preference.

In guppies, like most animals, females are choosy about who to mate with, since they invest more in their offspring than males, which don’t help care for them. They tend to prefer males with striking colour patterns and big tails, traits that have been linked to good foraging ability and health. By choosing a male with these qualities, female guppies give their offspring a good chance of inheriting the same useful traits.

Despite this, females often go on to make different choices. Alberto Corral López and colleagues at Stockholm University wanted to find out if brain size could account for this.

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Colourful males

Corral López and his team tested 36 females bred to have large brains, 36 bred to have small brains, and 16 females similar to guppies found in the wild. Previous studies have shown that large-brained guppies perform better in cognitive tests, suggesting that they are smarter.

Each female was given the opportunity to associate with two males, one more colourful than the other. Females are known to spend more time close to males they would prefer to mate with, so the team timed how long they spent with each male.

The wild-type and large-brained females both showed strong preferences for more colourful males, while small-brained females showed no preference. This couldn’t be explained by differences in colour perception, as tests showed that small-brained and large-brained females could perceive colour equally as well.

This suggests that more complex cognitive abilities are involved in deciding which males to associate with, says Corral López. “It’s not sensory input but how they process the information that they receive from the different males.”

Memory of mates

In the test, the females could swim around to spend time with either male, but they weren’t next to each other. This meant that the females couldn’t directly compare the males, instead having to use their memory to decide which one they preferred.

The test took place over a short period of time, but females may need to remember what the males they have met look like for much longer in the wild, to help them decide how attractive they find new individuals.

It’s not clear what cognitive abilities are required to choose a good mate, but if females need to pay attention to several different factors, it makes sense that large brains would be helpful, says Amanda Ridley, an evolutionary biologist at the University of Western Australia. “I would imagine that where some complicated processing of multimodal information is required, females might benefit from having bigger brains to allow them to make better mate choice decisions.”

But Neeltje Boogert at the University of Exeter, UK, says breeding for different brain sizes in the females might have affected their appearance or behaviour in ways that in turn changed the males’ behaviour. “I think the role of such a behavioural feedback loop cannot be excluded,” she says.

She also points out that nature tends to select for behaviours, such as making accurate decisions, rather than brain size itself. “I think it would be very interesting to perform these selection experiments in the opposite direction: if you select on females that strongly prefer ‘more attractive’ males, how does their associated neuronal structure change as a result?”

While plenty of studies have found that brainpower can make animals, including humans, more attractive to potential partners, few have looked at the cognitive demands for those doing the choosing. Brain tissue takes a lot of energy to run, so growing a large brain has considerable costs for animals.

“What we suggest is that being able to choose a better male and the fitness benefits you acquire compensates for the energy required to grow a larger brain, from an evolutionary point of view,” says Corral López.